Presently, there does not exist any apparatus to accurately measure the lowest limit of the useful water layer (UL), i.e., the humidity level of the soil from which the plant starts off a reduction in its production or "Productivity Depression Point (PDP)."
While growing crops based on irrigation techniques, two common questions are relevant: how much and when to irrigate? The answers have to be given instantly. As for the first one, an instrument has been recently developed to accurately answer the question in a few seconds. It is the HIDROCAPACITOR Yo (U.S. Pat. No. 8,903,754)/
The second question--when to irrigate? Water must be added to the soil when the potential of- such water is still sufficiently high (less negative), without exposing the plant to a water need that may affect its development and when the hydraulic conductivity of the soil is high enough (as per Darcy's equation) to meet the evapo-transpirative demand. Water must be supplied to the right quantity, avoiding losses either by superficial flow-off or by deep drainage. In practical terms, this criterion is usually simplified by meeting the conditions of each particular case. In an extreme case, the farmer irrigates the crop at his own discretion based on a subjective criterion acquired by experience. In case of a rational operation--when an optimization of the quantity of water is intended--adding water to the soil must obey a more scientific criterion.
The concept of water availability is complex and presents a big variability from situation to situation. All in all, however, most plants have their growth affected when the potential matrix of the soil water reaches values under -5 atm. Thus, to avoid water "STRESSING", the irrigation is recommended when the potential of the water in the soil reaches values close to -8 atm.
Yet, each type of crop has an optimum water potential interval for each growth stage.
In the lack of equipment or instrumentation to determine the critical humidity (PDP), experimental data is utilized. Such data, however, whose validity is arguable, are only valid for those hedapho-climatical conditions and the variety for which they were established.
For the temperate regions, there already exist experimental data for many crops. The estimates are obtained through experiments by submitting the crop to different levels of humidity and by verifying from which level of humidity the decline in production starts to occur. The method is rather complex, expensive and toilsome, with its results considered to be valid if restricted to the areas and conditions where they were obtained.
Aiming at the establishment of a simple, quick and economic procedure to eliminate the faults and errors in the estimates of the above described PDP, the present invention was developed.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.